Transistors in which an active layer is formed using a semiconductor thin film which is formed over a substrate having an insulating surface are widely used in electronic devices such as integrated circuits (ICs) and image display devices (display devices).
Conventional thin film transistors mostly have what is called a planar structure, in which a semiconductor film, an insulating film, an electrode, and the like are stacked over a plane. Owing to advances in manufacturing processes which enable miniaturization of such transistors, channel width is reduced, which causes a reduction in on-state current. Therefore, in recent years, a transistor having a novel structure which is an alternative to the conventional planar structure has been developed. For example, Patent Document 1 discloses a fin-type transistor in which a polycrystalline silicon film is used as an active layer (referred to as a semiconductor thin film in Patent Document 1).